There has recently been a strong demand for automobile steel sheets to be increased in strength from the viewpoint of enhanced fuel efficiency which leads to environment conservation. To cope with tighter restrictions of carbon dioxide emissions, automobile manufacturers have considered the use of steel sheets exhibiting a tensile strength in excess of 1270 MPa. Further reduction in the thickness of steel sheets has been demanded from the viewpoint of making more lightweight parts, and there has been an increasing need for thin steel sheets having a sheet thickness of 0.8 to 1.6 mm. In general, it is impossible to form ultra high strength cold rolled steel sheets with a tensile strength of 1270 MPa or more by methods such as drawing and stretching which are applicable to forming of mild steel sheets. Thus, bending and stretch flanging are main forming methods used for such ultra high strength cold rolled steel sheets. In the case where ultra high strength cold rolled steel sheets are used for the manufacturing of automobile structural parts, good bendability and stretch flangeability constitute important selection criteria. Further, ultra high strength cold rolled steel sheets with a tensile strength of 1270 MPa or more have a potential to suffer a delayed fracture. Thus, good delayed fracture resistance is another requirement.
As ultra high strength cold rolled steel sheets exhibiting good workability, dual phase steel sheets are known in which hard martensite has been dispersed in a soft ferrite phase to achieve both high strength and workability. The use of such steel sheets has been widespread. Indeed, although such dual phase steel sheets exhibit good ductility, they are poor in bendability and cannot be used for parts manufactured through severe bending. Further, the presence of soft ferrite makes it difficult to ensure a tensile strength exceeding 1270 MPa.
When a steel sheet is worked by bending, an outer peripheral superficial portion undergoes high tensile stress in a circumferential direction while an inner peripheral superficial portion is highly compressively stressed. Thus, the state of superficial portions greatly affects the bendability of an ultra high strength cold rolled steel sheet. That is, it has been known that the provision of a soft superficial layer reduces the tensile and compressive stress applied to the surface when the steel sheet is worked by bending, thereby improving bendability. With regard to high strength steel sheets having a soft superficial layer, Japanese Unexamined Patent Application Publication Nos. 2-175839, 5-195149, 10-130782 and 2002-161336 disclose steel sheets and methods for the manufacturing thereof as described below.
JP '839, which is directed to improving bendability and spot weldability, discloses a high strength steel sheet whose surface layer has been decarburized and annealed and which includes a superficial soft layer representing 10 vol % and an inner, i.e., core, hard layer containing not less than 10 vol % of retained austenite, and a method for manufacturing such steel sheets. The core layer contains as much as 10 vol % or more of retained austenite. However, martensite is formed during forming and voids are generated in the boundaries between the hard phase and soft ferrite, with the result that cracks occur and propagate easily. Thus, such a high content of retained austenite can adversely affect bendability.
JP '149 discloses a cold rolled steel sheet which has superficial soft layers on both sides that represent 3 to 15% and contain C at not more than 0.1 wt %, and in which the remaining portion is a multi phase containing retained austenite at less than 10% as well as a low temperature transformation-forming phase or ferrite. JP '149 further discloses a method for manufacturing such steel sheets. However, the surface hardness of such a steel sheet is markedly decreased because of the superficial soft layers containing C at not more than 0.1 wt %, thus leading to a decrease in terms of fatigue properties. Further, JP '149 is silent with respect to delayed fractures.
JP '782 discloses a cold rolled steel sheet in which a superficial portion extending from each surface to a depth of 10 μm to 200 μm is based on ferrite, and the remaining inner portion is based on bainite and martensite, as well as a method for manufacturing such steel sheets. However, the ferrite-based superficial portions extending from the surface to a depth of 10 μm to 200 μm have a problem of poor fatigue properties.
JP '336 discloses a cold rolled steel sheet with excellent stretch flangeability in which the metal microstructure except portions extending from the surface to a depth of within 10 μm is substantially formed of a martensite single phase, as well as a method for manufacturing such steel sheets. Although JP '336 describes that ferrite may be sometimes formed in the superficial layers having a thickness of 10 μm or less, the disclosed technique is not such that superficial soft layers are formed positively while controlling the proportions of these layers so as to improve workability. Further, the disclosed steel sheet exhibits insufficient bendability.
As described above, there have been no ultra high strength cold rolled steel sheets which exhibit good bendability as well as high strength of 1270 MPa or more and also have excellent delayed fracture resistance.
It could therefore be helpful to provide an ultra high strength cold rolled steel sheet with a sheet thickness of 0.8 to 1.6 mm which exhibits excellent bendability and delayed fracture resistance.
We found that an ultra high strength cold rolled steel sheet with a small thickness which exhibits excellent bendability and tensile strength of not less than 1270 MPa as well as is excellent in terms of delayed fracture resistance after being formed can be obtained by controlling the composition of steel components within an appropriate range and controlling the microstructure.
We thus provide:                (1) An ultra high strength cold rolled steel sheet with excellent bendability which contains, in terms of mass %, C at 0.15 to 0.30%, Si at 0.01 to 1.8%, Mn at 1.5 to 3.0%, P at not more than 0.05%, S at not more than 0.005%, Al at 0.005 to 0.05% and N at not more than 0.005%, with the balance being represented by Fe and inevitable impurities, and has a steel sheet superficial soft portion satisfying Equations (1) and (2):Hv(S)/Hv(C)≦0.8  (1)                    wherein Hv(S) is the hardness of the steel sheet superficial soft portion, and Hv(C) is the hardness of a steel sheet core portion,0.10≦t(S)/t≦0.30  (2)            wherein t(S) is the thickness of the steel sheet superficial soft portion, and t is the sheet thickness,            the steel sheet superficial soft portion containing tempered-martensite at a volume fraction of not less than 90%, the microstructure of the steel sheet core portion including tempered-martensite,            the ultra high strength cold rolled steel sheet having a tensile strength of not less than 1270 MPa.                        (2) The ultra high strength cold rolled steel sheet with excellent bendability described in (1), which further contains, in terms of mass %, one or more selected from Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10% and V: 0.01 to 0.50%.        (3) The ultra high strength cold rolled steel sheet with excellent bendability described in (1) or (2), which further contains, in terms of mass %, B at 0.0001 to 0.005%.        (4) The ultra high strength cold rolled steel sheet with excellent bendability described in any one of (1) to (3), which further contains, in terms of mass %, one or more selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50% and Cr: 0.01 to 0.50%.        
Ultra high strength cold rolled steel sheets with a small thickness can be obtained which exhibit an ultra high tensile strength of not less than 1270 MPa and are excellent in terms of bendability and delayed fracture resistance. The ultra high strength cold rolled steel sheets can be used for the production of parts that are difficult to form, for example, automobile structural members, to which application of high strength steel sheets has been difficult. When our ultra high strength cold rolled steel sheet is used for automobile structural members, those steel sheets can contribute to the weight reduction as well as the safety enhancement for automobiles, thus achieving industrial advantages.